This invention relates generally to radio telephone systems and, more particularly, to cellular communication systems wherein user equipment (UE) operates within cells, and has a capability to perform cell selection and reselection operations.
In a cellular communication system a plurality of base stations or base sites are provided, each of which has a surrounding coverage area (cell) within which mobile stations, such as cellular telephones, personal communicators, packet data terminals and the like can operate. These various devices can be collectively referred to as user equipment (UE). When the UE is capable of motion relative to the cells, then it can travel from cell to cell. This requires cell-to-cell handover procedures. One aspect of handover is that the UE will normally have a capability to perform surrounding (neighbor) cell signal measurement, measurement reporting and, importantly, selection/reselection operations. An important goal is to ensure that the UE is always operating with a cell capable of servicing the communication needs of the UE.
One modern cellular communication system that is under development is known as the 3rd Generation Partnership Project (3GPP). It is desirable that the system specifications for the 3GPP system provide flexibility while, from the UE implementation point, a not too demanding cell reselection procedure. Inter-frequency measurements for cell reselection should be performed in such a way that it can be ensured that the UE can still correctly receive paging messages, without compromising the battery life, and hence the talk time and standby time, of the UE.
Currently, the 3GPP WG2 technical specification, TS25.304 V.3.0.0 (1999-10), does not define how the UE should measure and make cell reselection to other frequencies (cells on different layers or on different frequencies but on the same layer) efficiently from both the UE and the UMTS (Universal Mobile Telecommunications System) Terrestrial Radio Access Network (or UTRAN) point of view. This specification does set some priorities for measurements and cell reselection, but these priorities are not optimal in all respects.
The 3GPP WG2 technical specification, TS25.304 V.3.0.0 (1999-10) is incorporated by reference herein in its entirety.
In brief, the purpose of cell reselection is to regularly look for the best cell for the UE to camp on, and a current serving cell is changed when a better one is found. The criterion for a better cell is different for intra/inter-frequency and inter-radio access system reselections.
The following steps are carried out when evaluating cells for cell reselection:
(A) The candidate list of potential cells to camp on includes the cells for intra/inter-frequency and intra-radio access system reselections in system information of the serving cell.
(B) Calculate a Quality (Q) value for each cell and a Cell Selection (S) value for each current UMTS Terrestrial Radio Access (UTRA) mode cell in the candidate list.
(C) Depending on which type of cells are on the candidate list (intra-frequency, inter-frequency and inter-radio access system), select the cell that best fulfills the corresponding criteria.
Better cells are prioritized in the following order when several cells fulfil their corresponding criteria:
(1) Intra-frequency neighboring cells
(2) Inter-frequency neighboring cells
(3) Inter-radio access system neighboring cells
The criteria for a better intra-frequency cell are found in Section 5.2.2.4.2 of the 3GPP WG2 technical specification, TS25.304 V.3.0.0 (1999-10):
Sn greater than 0
Qn greater than Qs+Qoffsets,n+Qhyst
The inter-frequency cell reselection evaluation uses the same criteria as intra-frequency cell reselections.
The cell selection value, S, is defined in Section 5.2.2.1.2 of the 3GPP WG2 technical specification, TS25.304 V.3.0.0 (1999-10) as:
S=Qxe2x88x92Qminxe2x88x92Pcompensation.
Changes were proposed to the 3GPP WG2 technical specification to provide new parameters in the inter-frequency cell reselection criteria in order to support hierarchical cell structures, with two cell layers, in the UE idle mode, and for cell selection and reselection in the connected mode (Change Request number 5 made to 25.304 version 3.0.0 (Ericsson), published Nov. 29,-Dec. 3, 1999 in 3GPP TSG-RAN WG2 Meeting #9 in Sophia Antipolis, France, and Change Request number 2 r1 made to 25.304 version 3.0.0 (Ericsson), published Nov. 29-Dec. 3, 1999 in 3GPP TSG-RAN WG2 Meeting #9 in Sophia Antipolis, France, respectively. The disclosures of each of these Change Request documents is incorporated by reference herein in its entirety.)
Also in the Change Request number 5 an optional Qhcs parameter is proposed for giving the operator an absolute threshold for controlling the cell radius of a micro-cell.
The inventors have realized that problems exist in the currently proposed techniques for providing a hierarchical cell structure, specifically with regard to the cell reselection procedures.
It is an object and advantage of this invention to provide an improved cell reselection procedure for use in a cellular communications network.
It is another object and advantage of this invention to provide an improved cell reselection procedure for use in a cellular communications network having a hierarchical cell structure.
It is another object and advantage of this invention to provide an improved cell reselection procedure for use in a hierarchical cell structure that is biased to cause the user equipment to reselect to smaller cells that are higher in the hierarchy, and wherein a speed of the user equipment is a consideration that can cause the user equipment to perform a reselection to a larger cell, as opposed to a smaller cell.
The foregoing and other problems are overcome and the objects and advantages of the invention are realized by methods and apparatus in accordance with embodiments of this invention.
In one aspect this invention provides a method for performing a cell reselection in a system comprised of a hierarchy of cells, wherein cells of one layer of the hierarchy have a size that differs from cells of another layer of the hierarchy. The method includes steps of (a) identifying to a user equipment a layer to which individual cells in a neighbor cell list are associated; (b) when performing neighbor cell measurements for reselection purposes, avoiding a measurement of cells in the list that are larger than a current serving cell unless a Cell Selection (S) parameter falls below a search threshold parameter (Ssearch), and is greater than zero; and (c), if S is less than or equal to zero, and no cell reselection to a better cell is in process, beginning a measurement of neighbor cells without regard for their hierarchical level.
If the user equipment has made cell reselection to N different cells on the same hierarchical layer within a time Tmax, the user equipment initiates a reselection procedure for larger neighbor cells. This procedure accommodates user equipment that is moving at a relatively high speed through the cells. If the user equipment locates a larger cell, which fulfills cell reselection criteria Sn greater than 0 and Qn greater than Qs+Qoffsets,n+Qhyst for time Treselection, cell reselection to the larger cell is made. Furthermore, if the user equipment performs a cell reselection to the larger cell, the user equipment does not attempt to reselect to a smaller cell within a time X, unless an immediate cell evaluation is triggered, wherein the time X can be pre-specified, or can be set by the network.